Fieldbuses are routinely used in process industries at field level for data exchange between field devices and a control system. This branch of industry is dominated by Foundation Fieldbus (FF) and PROFIBUS. Both buses are based on the same physical layer according to IEC61158-2.
In practice, several devices sometimes from different manufacturers are operated on one line or section in automation systems. In contrast to the previously predominant 4 . . . 20 mA technology, the planning, installation, startup and maintenance of fieldbus networks makes considerably higher demands on planners, installers and operators. Simple wiring faults like short circuit, wire break or reverses, such as previously occurred because of the high number of signals, are relatively easy to find and correct in fieldbus networks.
Much more critical are the consequences of incorrect layout, faulty installation and lack of monitoring of the operation of the system. Typical faults in the area of planning and layout are manifested in too great lead lengths, too many devices on one line or too weakly rated power supply.
Typical faults in the installation are, for example, lacking or inadequate equipotential bonding, or defects in shielding and grounding of the fieldbus cables.
An added difficulty is that both fieldbus protocols sometimes allow a lot of margin in the interpretation of conditions and parameters, which leads to “creeping” incompatibility between components.
While the aforementioned problems could be avoided by the user, incompatibilities between devices cannot be influenced by the user, nor can he rule them out.
Backup mechanisms in the protocols, such as data reconstruction or message repetition, will suppress most of the faults mentioned above and make them invisible for the user. In most cases, the system will begin operation after numerous corrections, without the operator knowing about the actual transmission quality and thus without guaranteed reliability. The faults normally increase with time, defects in planning and initial installation becoming visible especially in the event of installation extensions. However, these tend to be associated with the extension, as the system was not attracting attention previously.
If nothing odd occurs during startup, measurements are regularly dispensed with and the user relies on the fault tolerance of the devices. In the best case, the startup is monitored with a mobile bus monitor, and control is only transferred once the statistics are satisfactory. The measurements are limited in time and are performed by specialists.
Based on empirical knowledge, systems are planned generously, i.e. with considerable reserve, and the existing potential is thus far from exhausted. The networks are correspondingly complex.
As a result of the lack of continuous information about the transmission quality, the network is only analyzed when faults are detected, without planning. If the system is expanded, new networks are installed in order not to put existing functions at risk, as there is no sound information about the reserve in the network.